Magneto-optical storage media

ABSTRACT

A magneto-optical storage medium includes an interference layer, a magnetic domain expansion layer, an intermediate layer, a magnetic masking layer, a recording layer, and a protection layer which are sequentially formed on a substrate. The magnetic domain expansion layer produces a smaller frictional force due to wall coercivity than do the other magnetic layers. The intermediate layer has the Curie temperature T C2  which is lower than those of the other magnetic layers. The magnetic masking layer is in a perpendicular magnetization state at temperatures that are in a proximity of T C2 , and changes into an in-plane magnetization state at temperatures that are higher than the proximity of T C2 . The recording layer produces a higher coercive force than those produced by the magnetic domain expansion layer at room temperature.

This a Request for filing a divisional application under 37 C.F.R. §1.53(b) of prior application Ser. No. 09/440,929 filed on Nov. 16, 1999, nowU.S. Pat. No. 6,388,956 the entire contents of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention refers to magneto-optical storage media formagneto-optically storing and reproducing information with a laser beam.

BACKGROUND OF THE INVENTION

Magneto-optical storage media, an application of magneto-opticaleffects, are increasing their storage density as a result of a varietyof research and development projects to develop repeatedly rewritableinformation storage media with a large capacity.

The magneto-optical storage medium has a short-coming that reproductionproperties deteriorate with a relative decrease in the diameter orinterval of storage bits, which form magnetic domains for storage, tothe diameter of the light beam focused on the medium.

This is because the diameter of the light beam focused on a targetstorage bit encompasses an adjacent storage bit, and the informationstored on the individual storage bits cannot be separately reproduced.

To eliminate the short-coming, attempts have been made to improvestorage density through working on the arrangement and reproductiontechnique of the storage medium. One of the proposed methods is anexpanded magnetic domain reproduction system by means of displacement ofmagnetic walls.

Here, a reference is made to a prior art, Japanese Laid-Open PatentApplication No. 6-290496/1994 (Tokukaihei 6-290496 published on Oct. 18,1994; hereinafter will be referred to as Prior Art 1) disclosing anexpanded magnetic domain reproduction technology by means ofdisplacement of magnetic walls.

According to the technology, in a magneto-optical storage medium, highdensity storage is realized using reproduction signals having anincreased amplitude, by coupling magnetic films that form amulti-layered structure through an exchange force, and increasing tinystorage magnetic domains in a recording layer 104 in size by means of amagnetic domain expansion layer 101. FIG. 7(a) shows such anarrangement. Note that arrows are drawn in some layers to denote thedirections of sub-lattice magnetization of the transition metalscomposing the layers, and also that magnetic walls (Bloch walls) 110 areformed in the layers between such adjacent magnetic domains that thedirections of their magnetization are different from each other by 180°.The layers in which no arrows are drawn are non-magnetic. Portions ofthe magnetic layers in which arrows are absent denote loss of orderedmagnetization in them due to temperature elevated to the Curietemperature or even higher.

There are four principal requirements for the magneto-optical storagemedium as follows:

1. The recording layer 104 should be provided so as to stably hold tinymagnetic domains in place at temperatures ranging from room temperatureto temperatures reached during reproduction.

2. The recording layer 104, the intermediate layer 102, and the magneticdomain expansion layer 101 should be coupled through an exchange forceat least in a proximity of the Curie temperature, T_(C102), of theintermediate layer 102.

3. The intermediate layer 102 should lose ordered magnetization as itstemperature rises past the Curie temperature T_(C102), cutting off theexchange coupling among the recording layer 104, the intermediate layer102, and the magnetic domain expansion layer 101 above the Curietemperature T_(C102).

4. The magnetic domain expansion layer 101 should generate a lowfrictional force due to magnetic domain wall coercivity, and atemperature gradient should cause a magnetic wall energy gradient.Hence, the magnetic walls 110 move where the intermediate layer 102functions so as to cut off the exchange coupling, with the portion towhich magnetization is duplicated from a magnetic domain 104 a in therecording layer 104 as an original. As a result, the magnetization inthose regions become aligned to the same direction as that of themagnetic domain 104 a.

FIG. 7(b) is a graph illustrating the distribution of temperature in themiddle of a track of a disk moving to the right relative to the personobserving as a result of projection of a laser beam to themagneto-optical storage medium. Here, the disk is moving at such a highlinear velocity that temperature is highest downstream of the center ofthe beam spot with respect to the direction of the movement of the beamspot.

FIG. 7(c) is a graph illustrating the distribution of the magnetic wallenergy density δ101 in the magnetic domain expansion layer 101 in acircumferential direction. Typically, the magnetic wall energy densitydecreases with an increase in temperature, dropping to 0 above the Curietemperature. Therefore, when there is a temperature gradient in acircumferential direction as shown in FIG. 7(b), the magnetic wallenergy density δ101 decreases with high temperatures as shown in FIG.7(c).

The force, F₁₀₁, exerted on the magnetic walls in the layers at positionx along the circumference is given by the following expression:

F ₁₀₁ =−dδ ₁₀₁ /dx

The force F₁₀₁ acts to move the magnetic walls to a lower magnetic wallenergy level. The magnetic domain expansion layer 101, in comparison tothe other magnetic layers, generates a low frictional force due to wallcoercivity, i.e., is likely to allow movement of the magnetic walls.Therefore, when the exchange force is no longer available from theintermediate layer 102, the magnetic domain expansion layer 101 allowsthe force F₁₀₁ to move the magnetic walls to a lower magnetic wallenergy level.

In FIG. 7(a), prior to the projection of a laser beam to the disk, thethree magnetic layers are coupled through an exchange force wheretemperature is equivalent to room temperature, while the magneticdomains stored in the recording layer 104 have been duplicated to themagnetic domain expansion layer 101. Here, in each of the layers, thereexist magnetic walls between such adjacent magnetic domains that havemutually reverse magnetization directions.

Where temperature has been raised to the Curie temperature, T_(C102), ofthe intermediate layer 102 or higher, the intermediate layer 102 losesmagnetization, cutting off the exchange coupling between the magneticdomain expansion layer 101 and the recording layer 104; therefore themagnetic domain expansion layer 101 can no longer hold the magneticwalls in place, allowing the magnetic walls to move toward a highertemperature portion according to the force F₁₀₁ exerted on the magneticwalls. Here, the magnetic walls move at a velocity sufficiently fasterthan does the medium. Therefore, the duplicate magnetic domains in themagnetic domain expansion layer 101 are larger in size than those storedin the recording layer 104.

However, the medium described in Prior Art 1 entails following problems:since the exchange coupling from the recording layer 104 through themagnetic domain expansion layer 101 is cut off where temperature hasrisen to the Curie temperature, T_(C102), of the intermediate layer 102or higher, the magnetic walls become movable in the magnetic domainexpansion layer 101, whereas a parasitic magnetic field generated by thestorage magnetic domains of the recording layer 104 builds up anunignorable magnetostatic coupling force.

The magnetostatic force arising from the magnetic fields generated bythe other magnetic layers and the like, as well as that arising from themagnetic moments of those magnetic layers per se, is ignorably small incomparison to the exchange force, since an exchange force arises fromexchange of electrons between magnetic layers at their interface.However, when the exchange coupling is cut off as in the above case, themagnetostatic coupling force is no longer ignorable. According to asuper-resolution technology disclosed in Japanese Laid-Open PatentApplication No. 10-40600/1998 (Tokukaihei 10-40600; published on Feb.13, 1998), Japanese Laid-Open Patent Application No. 6-150418/1994(Tokukaihei 6-150418; published on May 31, 1994), and other documents,the magnetization direction of the reproduction layer is caused toconform to the magnetization direction of the recording layer by the useof magnetostatic coupling with the recording layer.

In other words, in the medium arrangement described in Prior Art 1, amagnetostatic coupling force arises from the magnetic field generated bythe recording layer 104 and exerted on the magnetic domain expansionlayer 101 so that the magnetization direction of the magnetic domainexpansion layer 101 aligns to the magnetization direction of therecording layer, interrupting movement of the magnetic walls in themagnetic domain expansion layer 101, presenting an obstacle in theexpansion of the magnetic domains. An embodiment of Prior Art 1suggests, as a solution, a method to attenuate the generated magneticfield by fabricating the magnetic layers from compensating compositions.

However, according to the method, since temperature in the beam spotdoes not have a uniform distribution, resulting in an unevendistribution of magnetization; consequently, a magnetic field isgenerated by the recording layer at least in some regions in the beamspot. In addition, in the embodiment of Prior Art 1, the intermediatelayer 102, interposed between the recording layer 104 and the magneticdomain expansion layer 101, is relatively thin at about 10 nm.Therefore, the, magnetic domain expansion layer 101 is acted upon by themagnetic field generated by the recording layer 104 at a very shortdistance. The closer the magnetic domain expansion layer 101 is to therecording layer 104, the greater magnetostatic coupling force themagnetic domain expansion layer 101 experiences; adverse effects of theforce, as a result, become increasingly unignorable, interrupting themovement of the magnetic walls in the magnetic domain expansion layer101.

SUMMARY OF THE INVENTION

The present invention has an object to offer a magneto-optical storagemedium that can suppress magnetic fields generated by a recording layer,realize satisfactory movement of magnetic walls and expansion ofmagnetic domains, and increase signal strength.

A magneto-optical storage medium in accordance with the presentinvention, in order to achieve the above object includes:

a recording layer in which a plurality of storage magnetic domains areformed;

a first intermediate layer for cutting off exchange coupling with therecording layer when temperature rises past a predetermined temperature;

a magnetic domain expansion layer in which magnetic walls move toward ahigher temperature portion so as to form expanded magnetic domains whenthe first intermediate layer cuts off the exchange coupling with therecording layer; and

a magnetic masking layer, provided between the recording layer and thefirst intermediate layer, that is coupled with the recording layerthrough an exchange force at temperatures that are not higher than thepredetermined temperature, and meanwhile cuts off magnetostatic couplingof the magnetic domain expansion layer with the recording layer attemperatures that are higher than the predetermined temperature.

With the arrangement, when temperature does not exceed the predeterminedtemperature, the recording layer, the magnetic masking layer, the firstintermediate layer, and the magnetic domain expansion layer are coupledwith each other through an exchange force, allowing the storage magneticdomains in the recording layer to be duplicated to the magnetic domainexpansion layer. Meanwhile, as temperature rises past the predeterminedtemperature, the first intermediate layer cuts off the exchange couplingbetween the recording layer and the magnetic domain expansion layer,causing the magnetic domain expansion layer to lose magnetic order. As aresult, the magnetic domain expansion layer can no longer hold themagnetic walls in place, allowing the magnetic walls to move toward ahigher temperature portion with those magnetic domains that have beenduplicated by the exchange coupling as an original. Hence, the duplicatemagnetic domains expand toward a higher temperature portion, formingexpanded magnetic domains in the magnetic domain expansion layer.

As temperature rises past the predetermined temperature, besides thatthe first intermediate layer cuts off the exchange coupling between therecording layer and the magnetic domain expansion layer as above, themagnetic masking layer cuts off the magnetostatic coupling between therecording layer and the magnetic domain expansion layer. This preventsthe magnetic field (parasitic magnetic field) generated by the recordinglayer from acting on the magnetic domain expansion layer and thuspresenting an obstacle to expansion of the magnetic domains attemperatures that are higher than the predetermined temperature;therefore, in the magnetic domain expansion layer, the magnetic wallsmove in a satisfactory fashion, forming expanded magnetic domainsaccurately and precisely.

The formation of the expanded magnetic domains increases reproductionsignal strength; reproduction signals are now obtainable with asufficiently large amplitude without being adversely affected by noiseeven if linear storage density is increased. As a result, thereliability of a reproduction operation increases dramatically.

For a fuller understanding of the nature and advantages of theinvention, reference should be made to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating principles of a magneto-optical storagemedium in accordance with the present invention.

FIG. 2 is a schematic cross-sectional view illustrating an arrangementof a magneto-optical storage medium of embodiment 1.

FIG. 3 is a drawing showing results from evaluation of reproductionproperties of the magneto-optical storage medium of embodiment 1.

FIG. 4 is a schematic cross-sectional view illustrating an arrangementof a magneto-optical storage medium of embodiment 2.

FIG. 5 is a schematic cross-sectional view illustrating an arrangementof a magneto-optical storage medium of embodiment 3.

FIG. 6 is a schematic cross-sectional view illustrating an arrangementof a magneto-optical storage medium of embodiment 4.

FIG. 7(a) to FIG. 7(c) are drawings illustrating reproduction principlesof a conventional magneto-optical storage medium using a magnetic domainexpansion technique of a magnetic wall moving type.

DESCRIPTION OF THE EMBODIMENTS

First of all, referring to FIG. 1, principles of the present inventionwill be explained.

FIG. 1 is a schematic cross-sectional view illustrating amagneto-optical storage medium in accordance with the present invention.The medium includes a first magnetic layer (magnetic domain expansionlayer 1), a second magnetic layer (intermediate layer 2 or firstintermediate layer), a third magnetic layer (masking layer 3 or magneticmasking layer), and a fourth magnetic layer (recording layer 4); thefour layers are deposited sequentially in this order. Arrows are drawnin some layers to denote the directions of sub-lattice magnetization ofthe transition metals composing the layers, and magnetic walls (Blochwalls) 10 are formed in the layers between such adjacent magneticdomains that the directions of their magnetization are different fromeach other by 180°. Portions of the magnetic layers in which arrows areabsent denote loss of ordered magnetization in them due to temperatureelevated to the Curie temperature or even higher.

According to the present invention, the masking layer (magnetic maskinglayer) 3 is interposed between the recording layer 4 and theintermediate layer 2, which is a feature that cannot be found inconventional technologies. The magnetic masking layer 3 has magneticproperties adjusted, so that it changes into a perpendicularmagnetization state at least in a proximity of the Curie temperature,T_(c2)) of the intermediate layer 2, establishing exchange coupling fromthe recording layer 4 through the magnetic domain expansion layer 1; andchanges into an in-plane magnetization state at temperatures that areabove the proximity of T_(c2).

In the medium, in a proximity of T_(c2), similarly to conventionaltechnologies, the storage magnetic domains are duplicated from therecording layer 4 to the magnetic domain expansion layer 1 by anexchange force. Meanwhile, the masking layer 3 is in an in-planemagnetization state where temperature is equal to T_(c2) or higher,thereby forming magnetic loops with the recording layer 4 andsuppressing the occurrence of a magnetic field in a directionperpendicular to the surface of the layer. Consequently, adverse effectsfrom a magnetostatic coupling force from the recording layer 4 throughthe magnetic domain expansion layer 1 can be reduced, removing theobstacle for movement of the magnetic walls.

Referring to the drawings, the following description will explain indetail specific embodiments in accordance with the present invention.

Embodiment 1

FIG. 2 is a drawing illustrating an arrangement of a magneto-opticalstorage medium of the present embodiment. An interference layer 16, afirst magnetic layer (magnetic domain expansion layer 1), a secondmagnetic layer (intermediate layer 2), a third magnetic layer (maskinglayer 3), a fourth magnetic layer (recording layer 4), and a protectionlayer 17 are formed sequentially on a transparent substrate 15.

The transparent substrate 15 may be made of glass, polycarbonate,norbornene resin, or the like. The layers constituting themagneto-optical storage medium can be deposited by, for example,continuous sputtering using a sputtering device, or vacuum deposition.Among others, the magnetic layers, 1, 2, 3, and 4 are formedsuccessively while maintaining the vacuum to establish exchange couplingbetween them.

The interference layer 16 is provided so as to enhance magneto-opticaleffects and to protect the magnetic layers, and made of a transparentdielectric material, such as Si₃N₄, AlN, AlSiN, SiO₂, SiO, ZnS, andMgF₂. The same materials as those for the interference layer 16 can beused for the protection layer 17 which is provided to protect themagnetic layers. The interference layer 16 and the protection layer 17do not play essential roles in the present invention, and their detaileddescription is therefore omitted here.

Further referring to FIG. 2, arrows are drawn in some layers to denotethe directions of sub-lattice magnetization of the transition metalscomposing the layers, and magnetic walls (Bloch walls) 10 are formed inthe layers between such adjacent magnetic domains that the directions oftheir magnetization are different from each other by 180°.

The recording layer 4 can be formed from a material that has a highperpendicular magnetic anisotropy and a coercive force and is suited toform tiny and stable storage bits, such as TbFeCo, DyFeCo, TbDyFeCo, andother rare-earth metal and transition metal amorphous alloys. Storedinformation is retained in the form of the upward or downward directionin the magnetic domains of the recording layer 4. Alternatively, therecording layer 4 may be fabricated from a perpendicular magnetizationfilm of garnet, Pt/Co, Pd/Co, etc. and arranged so as to allow magneticduplication of information to another layer. Note that at roomtemperature, the recording layer 4 has a higher coercive force than doesthe magnetic domain expansion layer 1.

The intermediate layer 2 is made of a rare-earth metal and transitionmetal amorphous alloy, similarly to the other magnetic layers, and is aperpendicular magnetization film having a Curie temperature T_(c2)specified to about 70° C., lower than those of the other magnetic layers1, 3, and 4. The intermediate layer 2 is coupled with the recordinglayer 4 through an exchange force at T_(c2) or lower temperatures. Thematerial for the intermediate layer 2 may be those having lower Curietemperatures among TbFeCo, TbDyFeCo, DyFeCo, TbFeAl, TbFeCoAl, TbDyFeAl,and DyFeAl, for example. If a magnetic attenuation element is to beused, Cu and other elements, as well as Al, may be used.

The magnetic domain expansion layer 1 is made of a rare-earth metal andtransition metal amorphous alloy with a small perpendicular magneticanisotropy, such as GdCo, GdFeCo, GdFe, NdGdFeCo, or a bubble memorymaterial, such as garnet, so that in comparison to the other magneticlayers the magnetic domain expansion layer 1 produces a low frictionalforce due to wall coercivity, i.e., is likely to allow movement of themagnetic walls.

The masking layer 3 is made of a rare-earth metal and transition metalamorphous alloy; preferred examples include GdCo, GdFeCo, TbFeCo,DyFeCo, GdTbFeCo, GdDyFeCo, and TbDyFeCo. The masking layer 3 changesinto a perpendicular magnetization state at least in a proximity of theCurie temperature, T_(c2), of the intermediate layer 2 so as to becoupled with the recording layer 4 through an exchange force, and hasmagnetic properties such that it changes into an in-plane magnetizationstate at temperatures that are above the proximity of T_(c2).

At temperatures that are in a proximity of the Curie temperature T_(c2),the masking layer 3 is perpendicularly magnetized and therefore coupledwith the other magnetic layer 1, 2, and 4 through an exchange force,allowing duplication of the storage magnetic domains in the recordinglayer 4 to the magnetic domain expansion layer 1. At temperatures thatare above the proximity of T_(c2), the masking layer 3 changes into anin-plane magnetization state, forms magnetic loops with the recordinglayer 4, and serves to substantially mask the parasitic magnetic fieldgenerated by the recording layer 4. Conventionally, without the maskinglayer 3, the parasitic magnetic field was existent. Moreover, tofacilitate storage in the recording layer 4, the Curie temperature,T_(c3), of the masking layer 3 is preferably specified to a lower valuethan the Curie temperature, T_(c4), of the recording layer 4(T_(c3)<T_(c4)).

Note in FIG. 2 that the arrows (directions of the transition metalsub-lattice magnetization) in regions of the masking layer 3 that are inan in-plane magnetization state point in the same in-plane direction.However, this is only for the purpose of easy identification of thein-plane magnetization state; in fact, the transition metal sub-latticemagnetization do not necessarily point the same directions, since itforms magnetic loops with some of the storage magnetic domains of therecording layer 4. In the present invention, the masking layer 3 isprovided for a purpose of suppressing the magnetic field generated bythe recording layer 4 by means of the formation of magnetic loops. Thesame holds true throughout the drawings for the embodiments 2, 3, and 4.

Further, the masking layer 3 needs to have the aforementioned magneticproperties only in the multilayered arrangement of the presentembodiment, and may always be in-plane magnetized as a single layer.

As to the thicknesses of the layers, preferably, the interference layer16 is about 70 nm, the magnetic domain expansion layer 1 about 30 nm,the intermediate layer 2 about 10 nm, the masking layer 3 about 20 nm,the recording layer 4 about 40 nm, and the protection layer 17 about 20nm, for example. Note that the thickness of the interference layer 16 isdetermined so as to optimize multi-interference carried out with anevaluation device that is used in the present invention to conductevaluation. The evaluation device produces a laser beam of 680 nm inwavelength. If a different wavelength is to be adopted, the thickness ofthe interference layer 16 needs to be adjusted so as to produce optimumresults for that particular wavelength. The thicknesses introduced hereare mere examples, and are by no means limited to any particular valuesprovided that the present embodiment achieves its purposes as detailedhere.

Further, thermal properties may be also adjusted by further adding ametallic layer made of, for example, Cu and an Al alloy, such as Al,AlTa, AlTi, or AlCr to the arrangement. Further, protective coating maybe provided using polymer resin. Layers may be formed on substratesbefore combining the substrates together.

Next, the storage and reproduction operation of the magneto-opticalstorage medium of the present embodiment will be explained.

(Storage Operation)

A data signal is stored on the magneto-optical storage medium inaccordance with the present invention during rotation of the medium,either by projecting a laser beam having such a power to raise thetemperature of the recording layer 4 substantially to the Curietemperature T_(c4) and simultaneously modulating an external magneticfield or by applying a magnetic field of a fixed direction andsimultaneously modulating the laser power. In the latter case, storagemagnetic domains can be formed of a size smaller than the beam spotdiameter by adjusting the intensity of the laser beam so that only apredetermined region in a beam spot reaches T_(c4). As a result, asignal having a shorter cycle than the diffraction limit of light can bestored.

(Reproduction Operation)

A signal is reproduced during rotation of the medium, by projecting alaser beam having such a power to raise the temperature of themagneto-optical storage medium to a predetermined temperature that ishigher than the Curie temperature T_(c2).

In FIG. 2, prior to the projection of a laser beam to the disk, the fourmagnetic layers are coupled together through an exchange force wheretemperature is equivalent to room temperature, while the magneticdomains stored in the recording layer 4 have been duplicated to themagnetic domain expansion layer 1. Here, in each of the layers, thereexist magnetic walls 10 between such magnetic domains that have mutuallyreverse magnetization directions.

Where temperature has been raised to the Curie temperature, T_(c2), ofthe intermediate layer 2 or higher as a result of the projection of alaser beam, the intermediate layer 2 loses magnetization, and theexchange coupling is cut off between the magnetic domain expansion layer1 and the recording layer 4; therefore, the magnetic domain expansionlayer 1 can no longer hold the magnetic walls in place, allowing themagnetic walls to move toward a higher temperature portion according tothe force F₁ exerted on the magnetic walls, which corresponds to theforce F₁₀₁ in an aforementioned conventional example. Here, the maskinglayer 3 is in an in-plane magnetization state where temperature is equalto T_(c2) or higher, thereby forming magnetic loops with the recordinglayer 4 and suppressing the generation of a magnetic field by therecording layer 4.

Hence, the magnetostatic coupling established by the recording layer 4through the magnetic domain expansion layer 1 can be cut off. As aresult, the magnetic walls are released from the grip by a magnetostaticforce, as well as from the grip by an exchange force, and can move moresmoothly.

Here, the magnetic walls move at a velocity sufficiently faster thandoes the medium. Therefore, the duplicate magnetic domains in themagnetic domain expansion layer 1 are larger in size than those storedin the recording layer 4.

As mentioned above, in the case of using a magneto-optical storagemedium in accordance with the present invention, a magnetic domain 4 a,found near the front edge of the beam spot, that has a temperature in aproximity of T_(c2) can be expanded in the beam spot and duplicated forreproduction, enabling reproduction using reproduction signals of alarge amplitude; therefore, reproduction signals can be produced with asufficiently large amplitude without being affected by opticaldiffraction limit even if linear storage density is increased.

In the description so far on the movement of the magnetic walls,attention has been paid to the middle part of the track and its crosssections; however, since the actual track has a fixed width and atemperature gradient along the disk radius, we should also considermovement of the magnetic walls in such directions. Specifically, weshould consider restraining crosstalk, i.e., signal interference, whichincreases with the movement of the magnetic walls along the disk radiusand the resultant expansion of storage magnetic domains in an adjacenttrack. To do so, the movement of the magnetic walls needs to berestricted at the boundary between tracks.

This is done by a technique to modify the rectangle guide trench on thesubstrate, which are typically used to identify tracks of themagneto-optical storage medium, so that they are deeper thanconventional ones, for example, about 100 nm or even deeper. Anothertechnique is to store and reproduce information on the land which servesas an information track and separate the tracks by grooves.

According to the technique, the groove separates magnetic layersdeposited on the land as well. Alternatively, information is stored andreproduced on the groove which serves as an information track, and thetracks are separated by lands. Further, information may be stored andreproduced on both the land and the groove, and the tracks are separatedby guide trench boundaries, i.e., stepped portions. In practice,deposition takes place a little at the stepped portions to form a filmlinking magnetic layers; the film, however, is extremely thin comparedto other parts, and the magnetic coupling at the stepped portions of theguide trench is ignorable.

A further alternative, when the substrate has a U-shaped guide trench,is to subject the portion of a magnetic layer corresponding to the guidetrench to annealing by means of projection of a high powered laser so asto alter magnetic properties into in-plane magnetization, and hencemagnetically separate tracks from adjacent ones.

Provided that the track is magnetically separated from adjacent ones asabove, even if the storage magnetic domain is expanded to cover thewhole width of the track, the magnetic wall does not move crossing theboundaries with adjacent tracks; consequently, along the circumferenceof the disk, the storage magnetic domain is only expanded not exceedingthe width of a single track. Crosstalk (signal interference) from thestorage magnetic domains in adjacent tracks can be thus eliminated. Inaddition, since no magnetic walls emerge or vanish at the boundarieswith adjacent tracks, the magnetic walls can readily move along thetrack.

Further, the magnetic wall can move smoothly if after the formation ofthe interference layer 16 in manufacture of the magneto-optical storagemedium in accordance with the present invention, the surface of thelayer is smoothed by means of heating through the projection of Ar ionsor by means of a sputtering etching process. This further increases thelinear velocity of the medium.

Results from a measurement of properties of the magneto-optical storagemedium explained in the present embodiment 1 are presented in thefollowing.

A medium was fabricated as an example 1, such that the interferencelayer 16=70 nm, AlN; the magnetic domain expansion layer 1=30 nm, GdFeCo(Curie temperature T_(C1)>300° C.); the intermediate layer 2=10 nm,TbFeCo (T_(C2)=70° C.); the masking layer 3=20 nm, GdFeCo (T_(C3)=200°C.); the recording layer 4=40 nm, TbFeCo (T_(C4)=230° C.); and theprotection layer 17=20 nm, AlN.

Further, for comparative purposes, another medium was fabricated with nomasking layer included, as a comparative example 1, such that theinterference layer 16=70 nm, AlN; the magnetic domain expansion layer1=30 nm, GdFeCo (T_(C1)>300° C.); the intermediate layer 2=10 nm, TbFeCo(T_(C2)=70° C.); the recording layer 4=40 nm, TbFeCo (T_(C4)=230° C.),and the protection layer 17=20 nm, AlN.

The evaluation was conducted using an evaluation device that produces alaser beam of 680 nm in wavelength.

First of all, mark length dependency was measured on the example 1 andthe comparative example 1 to evaluate the expansion and reproduction ofmagnetic domains. The CNR mark length dependency here represents thesignal-to-noise ratio (C/N ratio) when a storage magnetic domains aresuccessively formed of a length that corresponds to a mark length, witha storage magnetic domain pitch being equal to double the mark length.The length of the storage magnetic domains was altered from 0.5 μm to0.2 μm (FIG. 3). The graph shows that storage magnetic domains, each 0.2μm long, could produce a reproduction property of about 40 dB.

Accordingly, samples fabricated including a masking layer 3 of athickness varying from 0 nm to 100 nm were evaluated. Results from theevaluation of signal properties on storage magnetic domains of 0.2 μmare shown in Table 1.

As understood from Table 1, in comparison to the comparative example 1,the example 1 increased the maximum value of C/N by about 2.0 dB, andexpanded the tolerable range of reproduction power to produce a C/Nratio of 40 dB or greater by about 5%. Further, better reproductionsignal properties than in conventional cases could be obtained withmasking layers 3 with thicknesses up to 60 nm.

This is presumably for the following reasons: (1) The magnetostaticcoupling between the recording layer 4 and the magnetic domain expansionlayer 1 could be cut off by the masking layer 3 in the example 1,whereas the coupling could not be cut off in the comparative example 1;therefore the magnetic walls in the magnetic domain expansion layer 1moved more smoothly in the example 1. (2) Since temperature distributionconditions became less demanding in enabling the magnetic walls to move,the maximum value of C/N rose, and the tolerable range of reproductionpower could be expanded. In other words, even higher density wasachievable without altering the conditions.

TABLE 1 Thickness of Masking Layer 3 C/N (dB) Remarks 0 41.0 ComparativeExample 1 10 42.0 20 43.0 Example 1 30 43.0 60 41.5 100 28.0

Note in the present embodiment 1 that since the Curie temperature,T_(C3), of the masking layer 3 was set lower than the Curie temperature,T_(C4), of the recording layer 4 (T_(C3)<T_(C4)), storage sensitivity ina storage operation did not deteriorate in comparison to the comparativeexample 1, and similar storage properties could be obtained. This isbecause the temperature of the medium reached a proximity of T_(C4) andcaused the masking layer 3 to lose ordered magnetization while data wasbeing stored, and therefore the ordered magnetization did not present anobstacle in storing data by means of magnetization inversion.

Embodiment 2

FIG. 4 is a schematic cross-sectional view of a magneto-optical storagemedium of the embodiment 2 in accordance with the present invention.

In the present embodiment 2, the same arrangement as that in theembodiment 1 is employed except that a fifth magnetic layer (secondintermediate layer 5) is interposed between the masking layer 3 and therecording layer 4. To avoid confusion, the intermediate layer 2described in the embodiment 1 will be renamed and referred to as thefirst intermediate layer 2.

The second intermediate layer 5 has magnetic properties similar to thoseof the first intermediate layer 2, and made of material similar to thatfor the first intermediate layer 2. Employment of a common materialwould contribute to an increase in efficiency in layer formation;however, different materials may be used provided that the purposes laidout below are achievable. In the present embodiment 2, both the firstintermediate layer 2 and the second intermediate layer 5=10 nm, TbFeCo.

Similarly to the first intermediate layer 2 already discussed in theembodiment 1, the second intermediate layer 5 establishes the exchangecoupling from the recording layer 4 through the masking layer 3, and hasa Curie temperature T_(C5) which is substantially equal to T_(C2). Ifthe medium incorporates this arrangement, storage magnetic domains canbe duplicated from the recording layer 4 to the magnetic domainexpansion layer 1 by means of the exchange coupling at temperatures thatdo not exceed T_(C2), and magnetostatic coupling can be suppressed bythe masking layer 3 at regions where temperature is T_(C2) or higher.Further, the magnetic loops formed by the recording layer 4 and themasking layer 3 at regions where temperature is T_(C2) or higher becomemagnetostatic through the interposition of the second intermediate layer5 that has lost ordered magnetization, and the magnetic field generatedby the recording layer 4 are caused to form loops and hence suppressed.

There are more advantages: if the Curie temperature, T_(C3), of themasking layer 3 is set higher than the Curie temperature, T_(C4), of therecording layer 4 (T_(C3)>T_(C4)), the masking layer 3 still retainsordered magnetization when data is being stored (temperature is in aproximity of T_(C4) or higher); however, since the recording layer 4 andthe masking layer 3 sandwich the second intermediate layer 5 which haslost ordered magnetization, the recording layer 4 neither is coupledwith the masking layer 3 through an exchange force nor is adverselyaffected by the masking layer 3, enabling data to be stored with a weakstorage magnetic field. In other words, there are less restrictionsimposed on the Curie temperature, T_(C3), of the masking layer 3, whichallows more freedom in the design of the media. Specifically, raisingthe Curie temperature T_(C3) increases the magnitude of the in-planemagnetization of the masking layer 3, better suppresses a magnetostaticcoupling force, and realizes mask effects.

Note that the Curie temperature, T_(C5) of the second intermediate layer5 is set substantially equal to T_(C2) in the present the embodiment 2;however, if an improvement in storage sensitivity is all that isrequired, the Curie temperature, T_(C5), of the second intermediatelayer 5 only needs to be lower than the Curie temperature, T_(C4), ofthe recording layer 4 so that the second intermediate layer 5 losesordered magnetization at a storing temperatures in a proximity ofT_(C4).

The following description will present results from a measurement ofproperties of the magneto-optical storage medium discussed in thepresent the embodiment 2.

A medium was fabricated as an example 2, such that the interferencelayer 16=70 nm, AlN; the magnetic domain expansion layer 1=30 nm, GdFeCo(T_(C1)>300° C.); the first intermediate layer 2=10 nm, TbFeCo(T_(C2)=70° C.); the masking layer 3=20 nm, GdFeCo (T_(C3)=200° C.); thesecond intermediate layer 5=10 nm, TbFeCo (T_(C5)=70° C.); the recordinglayer 4=40 nm, TbFeCo (T_(C4)=230° C.); and the protection layer 17=20nm, AlN. Further, another medium was fabricated according to the samearrangement, including a second intermediate layer 5 of a varyingthickness. Results from the evaluation of signal properties with storagemagnetic domains of 0.2 μm are shown in Table 2 in the same manner as inthe embodiment 1.

As understood from Table 2, in comparison to the comparative example 1,the example 2 increased the maximum value of C/N by about 3.0 dB, andexpanded the tolerable range of reproduction power to produce a C/Nratio of 40 dB or greater by about 5%. Further, better reproductionsignal properties than in conventional cases could be obtained withsecond intermediate layers 5 with thicknesses up to 60 nm.

TABLE 2 Thickness of Second Intermediate layer 5 (nm) C/N (dB) Remarks 041.0 Comparative Example 1 (No masking layer) 0 43.0 Example 1 (NoSecond Intermediate Layer) 10 44.0 Example 2 30 43.5 50 41.5 100 27.0

This is presumably for the following reasons: (1) The magentostaticcoupling between the recording layer 4 and the magnetic domain expansionlayer 1 could be cut off in the example 2, whereas the coupling couldnot be cut off in the comparative example 1; therefore the magneticwalls in the magnetic domain expansion layer 1 moved more smoothly inthe example 2. (2) Since the Curie temperature, T_(C3), of the maskinglayer 3 can be set high even in comparison to the example 1,magnetization is great, and strong mask effects could be obtained.Therefore, the maximum value of C/N rose as mentioned earlier, as wellas temperature distribution conditions became somewhat less demanding inenabling the magnetic walls to move; as a result, the tolerable range ofreproduction power could be expanded.

As discussed above, with the arrangement of the embodiment 2, moresatisfactory properties could be obtained, and even higher density wasachievable.

Embodiment 3

FIG. 5 is a schematic cross-sectional view showing a magneto-opticalstorage medium of the embodiment 3 in accordance with the presentinvention.

In the present embodiment 3, the same arrangement as that in theembodiment 1 is employed except that a reproduction layer 6 that iscoupled with the magnetic domain expansion layer 1 through an exchangeforce is provided on the light entering side of the magnetic domainexpansion layer 1. Specifically, a medium was fabricated such that theinterference layer 16=70 nm, AlN; the reproduction layer 6=20 nm, GdFeCo(Curie temperature T_(C6)>300° C.); the magnetic domain expansion layer1=30 nm, GdFeCo (Curie temperature T_(C1)>160° C.); the intermediatelayer 2=10 nm, TbFeCo (T_(C2)=70° C.); the masking layer 3=20 nm, GdFeCo(T_(C3)=200° C.); the recording layer 4=40 nm, TbFeCo (T_(C4)=230° C.);and the protection layer 17=20 nm, AlN.

The reproduction layer 6 is preferably made of a material that producesa large Kerr rotation angle during reproduction and, that, for thispurpose among others, has a high Curie temperature T_(C6). Specifically,to obtain better reproduction signal properties than those of themagnetic domain expansion layer 1, the reproduction layer 6=20 nm,GdFeCo (T_(C6)>300° C., T_(C6)>T_(C1)) in the present embodiment 3;however, other materials may be used provided that the purposes laid outbelow are achievable.

As a magnetic property of the magnetic domain expansion layer 1, theCurie temperature T_(C1) is set lower than a maximum temperaturereachable during reproduction so that the magnetic domain expansionlayer 1 loses ordered magnetization in a proximity of that highesttemperature during reproduction. As for magnetic properties of thereproduction layer 6, the reproduction layer 6 is magnetized in the samemagnetization direction as the magnetic domain expansion layer 1 byexchange coupling when the magnetic domain expansion layer 1 has orderedmagnetization, and is in an in-plane magnetization state in regionswhere the magnetic domain expansion layer 1 has lost orderedmagnetization. Note that the reproduction layer 6 needs to have thosemagnetic properties only when the magnetic layers are stacked, and thereproduction layer 6, as a single layer, may always be in an in-planemagnetization state.

The present arrangement offers the following advantages.

As already mentioned in the embodiments 1 and 2, the masking layer 3 isable to suppress the magnetic field generated by the recording layer 4,allowing smooth movement of the magnetic walls in the magnetic domainexpansion layer 1. Further, when the magnetic domain expansion layer 1expands the magnetic domains, those magnetic domains having temperaturesthat are in a proximity of the Curie temperature, T_(C2), of theintermediate layer 2 are expanded; those points in a single track thathave temperature equal to T_(C2) exist both at the front end, 4 a, ofthe beam spot (the back end when viewed from the medium) and at the rearend, 4 b, thereof (the front end when viewed from the medium), and themagnetic walls move from those ends toward a higher temperature portion.

In such an event, since the magnetic domain expansion layer 1 isspecified to lose ordered magnetization in regions where temperature isin a proximity of the maximum temperature, not all the duplicateexpanded magnetic domains in the reproduction layer 6 are found in thebeam spot; only the duplicate expanded magnetic domain at the front endof the beam spot is found in the beam spot. That is, only a certainsingle storage magnetic domain in the beam spot can be always expandedand duplicated for data retrieval during reproduction.

Moreover, in regions where the magnetic domain expansion layer 1 has atemperature equal to, or higher than, T_(C1) and therefore has lostmagnetization, the reproduction layer 6 is in an in-plane magnetizationstate, and masks magneto-optical signals; therefore, noise undesirablefor reproduction is masked.

Further, since the reproduction layer 6 is specified to have a greatKerr rotation angle, high quality magneto-optical signals can bereproduced. In addition, since data is not directly reproduced from themagnetic domain expansion layer 1, those materials that produce a largeKerr rotation angle are not necessarily used, the Curie temperature isnot necessarily set high, and media can be freely designed with a lowfrictional force due to wall coercivity.

The following description will present results from a measurement ofproperties of the magneto-optical storage medium discussed in thepresent the embodiment 3.

A medium was fabricated as an example 3, such that the interferencelayer 16=70 nm, AlN; the reproduction layer 6=20 nm, GdFeCo (T_(C6)>300°C.); the magnetic domain expansion layer 1=30 nm, GdFeCo (T_(C1)>160°C.); the first intermediate layer 2=10 nm, TbFeCo (T_(C2)=70° C.); themasking layer 3=20 nm, GdFeCo (T_(C3)=200° C.); the recording layer 4=40nm, TbFeCo (T_(C4)=230° C.); and the protection layer 17=20 nm, AlN.

Results from an evaluation similar to those in the embodiments 1 and 2show that in comparison to the comparative example 1, the example 3increased the maximum value of C/N by about 4 dB, and expanded thetolerable range of reproduction power to produce a C/N ratio of 40 dB orgreater by about 10%. Therefore, according to the example 3, even higherdensity was achievable.

This is presumably for the following reasons: (1) The magnetostaticcoupling between the recording layer 4 and the magnetic domain expansionlayer 1 could be cut off in the example 3, whereas the coupling couldnot be cut off in the comparative example 1; therefore, the magneticwalls in the magnetic domain expansion layer 1 moved more smoothly inthe example 3. (2) Since the magnetic domain expansion layer 1 was notnecessarily made of a material that produced large magneto-opticaleffects even in comparison to the example 1, the Curie temperatureT_(C1) was not necessarily set high. In other words, such materialscould be used that the frictional force due to wall coercivity was smalland the magnetic walls readily moved, facilitating expansion of themagnetic domains. (3) Since the reproduction layer 6 produced greatmagneto-optical effects, enabled its Curie temperature T_(C6) to be sethigh, and allowed for choices in materials of which the reproductionlayer 6 was made; therefore, excellent signal properties wereobtainable.

Note, especially in the present embodiment 3, that the Curietemperature, T_(C3), of the masking layer 3 is set higher than the Curietemperature, T_(C1), of the magnetic domain expansion layer 1. That is,since the magnetic domain expansion layer 1 is magnetized and themasking layer 3 is in an in-plane magnetization state in all the regionswhere the magnetic walls can move, it is ensured that the magnetostaticcoupling to the magnetic domain expansion layer 1 can be suppressed.

Further, the scope of the present invention is by no means restricted tothe medium arrangement discussed in the embodiment 3. For example, thesame effects as those with the embodiment 2 can be obtained with amedium arrangement incorporating the medium arrangement discussed in theembodiment 3 and further incorporating a second intermediate layer 5interposed between a recording layer 4 and a masking layer 3 asdiscussed in the embodiment 2.

Embodiment 4

FIG. 6 is a schematic cross-sectional view of a magneto-optical storagemedium of the embodiment 4 in accordance with the present invention.

In the present embodiment 4, the same arrangement as that in theembodiment 3 is employed except that a second interference layer(transparent dielectric layer 7) is interposed between the magneticdomain expansion layer 1 and the reproduction layer 6. The transparentdielectric layer 7 has a light transmitivity similar to that of theinterference layer 16, and can be made of a material similar to that forthe interference layer 16 discussed earlier. Employment of a commonmaterial would contribute to an increase in efficiency in layerformation; however, different materials may be used provided that thepurposes laid out below are achievable. In the present embodiment 4, theinterference layer 16=70 nm, AlN; and the transparent dielectric layer7=20 nm, AlN.

The following description will explain roles of the transparentdielectric layer 7.

In FIG. 6, the transparent dielectric layer 7 cuts off the exchangecoupling between the magnetic domain expansion layer 1 and thereproduction layer 6, establishing magnetostatic coupling instead. Asalready mentioned, as expanded magnetic domains are duplicated to themagnetic domain expansion layer 1, magnetization aligns to the samedirection and causes a magnetic field to occur, which develops into amagnetostatic coupling magnetic field acting on the reproduction layer6.

The reproduction layer 6 is in a perpendicular magnetization state atroom temperature, and changes into an in-plane magnetization state abovea certain temperature. Therefore, the reproduction layer 6 is in anin-plane magnetization state and produces no magneto-optical signals inregions where temperature is increased by a reproduction beam, which isan opposite of conventional magnetostatic-coupling-type-super-resolutionsystems. In the case of the system of the present embodiment, tinystorage magnetic domains are not duplicated for retrieval; instead,those magnetic domains that are already expanded and duplicated to themagnetic domain expansion layer 1 are duplicated for retrieval by meansof magnetostatic coupling. Therefore, perpendicular magnetizationregions in the reproduction layer 6 can be provided in large size.

In other words, in a conventional magnetostatic-coupling-typesuper-resolution medium, tiny magnetic domains need to be duplicated bymeans of magnetostatic coupling without changing their size, whilemasking surrounding magnetic domains; therefore, perpendicularmagnetization needs to be controlled so as to cover a tiny area. Incontrast, in a medium in accordance with the present invention, thosemagnetic domains that are expanded and duplicated are furtherduplicated; therefore perpendicular magnetization regions in thereproduction layer 6 do not need to be provided in a confined area.

In other words, since expanded magnetic domains can be duplicated to thereproduction layer 6, reproduction signals are obtainable with a largeamplitude.

Moreover, in a conventional magnetostatic-coupling-type super-resolutionmedium, very small magnetic fields generated by tiny magnetic domainsare used to realize magnetostatic coupling; therefore the recordinglayer and the reproduction layer need to be located as close to eachother as possible without establishing exchange coupling. However, inthe arrangement in accordance with the present invention, large magneticfields generated by the magnetic domains that are already expanded inthe magnetic domain expansion layer 1 are used for the duplication tothe reproduction layer 6, allowing the thickness of the transparentdielectric layer 7 to be set to a somewhat large value. This enables useof both Kerr and Faraday effects of the reproduction layer 6, andoptimization of multi-interference of light so as to obtain reproductionsignals with a large amplitude.

It is earlier mentioned that the reproduction layer 6 and the magneticdomain expansion layer 1 have the same magnetization direction as aresult of magnetostatic coupling. Accordingly, if the polarity of thesub-lattice magnetization of the rare-earth metal and transition metalcomposing the reproduction layer 6 conforms to the polarity of thesub-lattice magnetization of the rare-earth metal and transition metalcomposing the magnetic domain expansion layer 1 at least at temperaturesin a range from room temperature to the Curie temperature of themagnetic domain expansion layer 1, the directions of sub-latticemagnetization also mutually align in the two layers.

For example, in the present embodiment 4, both the reproduction layer 6and the magnetic domain expansion layer 1 have such composition thatrare-earth metals contained in the layers show strong and overwhelmingsub-lattice magnetization in the above-mentioned temperature range. Thisenables the rotation angles of polarized light to align to a singledirection, the light being polarized by three magneto-optical effectsavailable for use with multi-interference in the present invention,i.e., the Kerr effects of the reproduction layer 6, the Faraday effectsof the reproduction layer 6, and the Kerr effects of the magnetic domainexpansion layer 1; therefore, even larger rotation angles areobtainable, and reproduction signals are obtainable with a largeamplitude.

As mentioned above, in the case of using a magneto-optical storagemedium in accordance with the present invention, a magnetic domain,found at the front end of the beam spot, that has a temperature in aproximity of T_(C2) can be expanded in the beam spot and duplicated forreproduction, enabling reproduction with reproduction signals of a largeamplitude; therefore, reproduction signals can be produced with asufficiently large amplitude without being affected by opticaldiffraction limit even if linear storage density is increased.

Further, since the magnetic domain expansion layer 1 is coupled withnone of the magnetic layers through an exchange force in regions wherethe magnetic walls move, the magnetic walls readily move; that is,magnetic domains are readily expanded.

In addition, as to a magnetostatic coupling force, since the maskinglayer 3 suppresses the magnetic field generated by the recording layer4, the reproduction layer 6 is acted on only by duplication magneticfields generated by the magnetic domains expanded in the magnetic domainexpansion layer 1, allowing reduction in noise from the magnetic fieldgenerated by the recording layer 4.

The following description will present results from a measurement ofproperties of the magneto-optical storage medium discussed in thepresent the embodiment 4.

A medium was fabricated as an example 4, such that the interferencelayer 16=70 nm, AlN; the reproduction layer 6=20 nm, GdFeCo (Curietemperature T_(C6)>300° C.); the transparent dielectric layer 7=20 nm,AlN: the magnetic domain expansion layer 1=30 nm, GdFeCo (Curietemperature T_(C1)=160° C.); the intermediate layer 2=10 nm, TbFeCo(T_(C2)=70° C.); the masking layer 3=20 nm, GdFeCo (T_(C3)=200° C.); therecording layer 4=40 nm, TbFeCo (T_(C4)=230° C.); and the protectionlayer 17=20 nm, AlN.

Results from an evaluation similar to those in the embodiments 1 and 2show that in comparison to the comparative example 1, the example 4increased the maximum value of C/N by about 5.5 dB, and expanded thetolerable range of reproduction power to produce a C/N ratio of 40 dB orgreater by about 10%.

This is presumably for the following reasons: (1) The magnetostaticcoupling between the recording layer 4 and the magnetic domain expansionlayer 1 could be cut off in the example 4, whereas the coupling couldnot be cut off in the comparative example 1; therefore, the magneticwalls in the magnetic domain expansion layer 1 moved more smoothly inthe example 4. (2) Since the magnetic domain expansion layer 1 was notnecessarily made of a material that produced large magneto-opticaleffects even in comparison to the example 1, the Curie temperatureT_(C1) was not necessarily set high. In other words, such materialscould be used that the frictional force due to wall coercivity was smalland the magnetic walls readily moved, facilitating expansion of themagnetic domains. (3) Since the reproduction layer 6 produced greatmagneto-optical effects, enabled its Curie temperature T_(C6) to be sethigh, and allowed for choices in materials of which the reproductionlayer 6 was made; therefore, excellent signal properties wereobtainable.

In addition, even in comparison to the example 3, reproduction signalscould be obtained with a large amplitude, using multi-interference oflight, by adjusting the thickness of the transparent dielectric layer 7.This is for the following reason: since the reproduction layer 6magnetostatically coupled with the magnetic domain expansion layer 1,the magnetic domain expansion layer 1 is coupled with none of themagnetic layers through an exchange force during the motion of themagnetic walls, facilitating the movement of the magnetic walls; inother words, magnetic domains were readily expanded.

For these reasons, according to the present example 4, the maximum valueof C/N rose, as well as temperature distribution conditions becamesomewhat less demanding in enabling the magnetic walls to move; as aresult, the tolerable range of reproduction power could be expanded.That is, even higher density was achievable without changing theconditions.

Further, a reflective layer provided, on top of the arrangement, betweenthe transparent dielectric layer 7 and the magnetic domain expansionlayer 1 efficiently reflects the light that has passed through thereproduction layer 6 as described earlier in the present embodiment 4;besides, by adjusting the thickness of the transparent dielectric layer7 so as to optimize multi-interference, more excellent signal propertiesare obtainable. The reflective layer is made of a material having a highreflectance: for example, Al; AlNi, AlTa, AlTi, AlCr, and other Alalloys; and Cu.

Note that the scope of the present invention is by no means restrictedto the medium arrangements discussed here. For example, on top of themedium arrangement discussed in the embodiment 4, a second intermediatelayer 5 may be interposed between a recording layer 4 and a maskinglayer 3 as discussed in the embodiment 2. The same effects as those withthe embodiment 2 can be obtained in such an event.

As detailed so far, according to the present invention, a masking layeris provided between a recording layer and an intermediate layer, leak ofmagnetic fluxes, i.e., noise when viewed in a different perspective,that originate at the recording layer and extend to the magnetic domainexpansion layer can be suppressed in regions where temperature is equalto, or exceeds, the Curie temperature of the intermediate layer,facilitating accurate and precise expansion of magnetic domains.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art intended tobe included within the scope of the following claims.

What is claimed is:
 1. A magneto-optical recording medium formed of afilm and having a plurality of information tracks, each informationtrack comprising: in the following order, a substrate; an interferencelayer; a magnetic domain expansion layer; a first intermediate layer; amagnetic masking layer; a recording layer; and a protection layer,wherein the recording layer has a plurality of storage magnetic domains;the first intermediate layer cuts off exchange coupling with therecording layer when temperature rises past a predetermined temperature;the magnetic domain expansion layer has magnetic walls which move towarda higher temperature portion so as to form expanded magnetic domainswhen the first intermediate layer cuts off the exchange coupling withthe recording layer; and the magnetic masking layer is located betweenthe recording layer and the first intermediate layer, and is coupledwith the recording layer through an exchange force at temperatures thatare not higher than the predetermined temperature, and meanwhile cutsoff magnetostatic coupling of the magnetic domain expansion layer withthe recording layer at temperatures that are higher than thepredetermined temperatures.
 2. A magneto-optical recording medium formedof a film and having a plurality of information tracks, each informationtrack comprising: in the following order; a substrate; an interferencelayer; a first magnetic layer; a second magnetic layer; a third magneticlayer; a fourth magnetic layer; and a protection layer, wherein thefirst magnetic layer produces a smaller frictional force due to wallcoercivity than do the other magnetic layers, the second magnetic layerhas a Curie temperature lower than do the other magnetic layers, thefourth magnetic layer is a perpendicular magnetization film producing ahigher coercive force at room temperature than does the first magneticlayer, the third magnetic layer is provided between the second magneticlayer and the fourth magnetic layer, the third magnetic layer is in aperpendicular magnetization state at least at temperatures that are in aproximity of the Curie temperature and changes into an in-planemagnetization state at temperatures that are above the proximity of theCurie temperature, and the first, second, third, and fourth magneticlayers are mutually coupled through an exchange force.
 3. Themagneto-optical storage medium as set forth in claim 1, wherein thepredetermined temperature is a Curie temperature of the firstintermediate layer, and the magnetic masking layer exhibitsperpendicular magnetization at temperatures that are in a proximity ofthe Curie temperature and exhibits in-plane magnetization attemperatures that are above the proximity of the Curie temperature. 4.An information reproducing method for reproducing information from amagneto-optical recording medium formed of a film and having a pluralityof information tracks, each information track including in the followingorder, a magnetic domain expansion layer, a first intermediate layer, amagnetic masking layer, a recording layer, a protection layer, whereinthe recording layer has a plurality of storage magnetic domains, thefirst intermediate layer cuts off exchange coupling with the recordinglayer when temperature rises past a predetermined temperature, themagnetic domain expansion layer has magnetic walls which move toward ahigher temperature portion so as to form expanded magnetic domains whenthe first intermediate layer cuts off the exchange coupling with therecording layer, and the magnetic masking layer is located between therecording layer and the first intermediate layer, and is coupled withthe recording layer through an exchange force at temperatures that arenot higher than the predetermined temperature, and meanwhile cuts offmagnetostatic coupling of the magnetic domain expansion layer with therecording layer at temperatures that are higher than the predeterminedtemperature, said method comprising the steps of: rotating the medium;projecting a light beam onto the rotating medium from the side of themagnetic domain expansion layer; forming a temperature distributionhaving a gradient in the direction in which the light beam movesrelative to the medium and having a region of temperatures higher thanthe Curie temperature of the first intermediate layer; moving a domainwall in the magnetic domain expansion layer which comes into the regionof temperatures when the medium rotates toward a side having highertemperature in the temperature gradient in the magnetic domain expansionlayer; and reproducing the information by detecting rays of the lightbeam.
 5. The magneto-optical storage medium as set forth in claim 1,wherein the magnetic masking layer has a lower Curie temperature thandoes the recording layer.
 6. The magneto-optical storage medium as setforth in claim 1, further comprising a second intermediate layer,located between the magnetic masking layer and the recording layer, thathas a lower Curie temperature than does the recording layer.
 7. Themagneto-optical storage medium as set forth in claim 1, furthercomprising a reproduction layer, provided on a light entering side ofthe magnetic domain expansion layer, that is coupled with the magneticdomain expansion layer through an exchange force.
 8. The magneto-opticalstorage medium as set forth in claim 7, further comprising a transparentdielectric layer, located between the magnetic domain expansion layerand the reproduction layer, for cutting off exchange coupling betweenthe magnetic domain expansion layer and the reproduction layer so as toestablish magnetostatic coupling between the two layers.
 9. Themagneto-optical storage medium as set forth in claim 7, wherein thereproduction layer and the magnetic domain expansion layer are made of arare-earth metal and transition metal respectively, and have the samepolarity of sub-lattice magnetization of at least at temperaturesranging from room temperature to a Curie temperature of the magneticdomain expansion layer.
 10. The magneto-optical storage medium as setforth in claim 8, further comprising a reflective layer provided betweenthe transparent dielectric layer and the magnetic domain expansionlayer.
 11. The magneto-optical storage medium as set forth in claim 2,wherein the third magnetic layer has a lower Curie temperature than thatof the fourth magnetic layer.
 12. The magneto-optical storage medium asset forth in claim 2, wherein the third magnetic layer has a thicknessin a range larger than 0 nm and not exceeding 60 nm.
 13. Themagneto-optical storage medium as set forth in claim 2, wherein a fifthmagnetic layer is located between the third magnetic layer and thefourth magnetic layer, a Curie temperature, T_(c5), of the fifthmagnetic layer being not lower than the Curie temperature, T_(c2), ofthe second magnetic layer and not higher than the Curie temperature,T_(c4), of the fourth magnetic layer.
 14. The magneto-optical storagemedium as set forth in claim 2, wherein the Curie temperature, T_(c3),of the third magnetic layer is higher than the Curie temperature,T_(c4), of the fourth magnetic layer.
 15. The magneto-optical storagemedium as set forth in claim 13, wherein the fifth magnetic layer has athickness in a range larger than 0 nm and not exceeding 50 nm.
 16. Themagneto-optical storage medium as set forth in claim 13, wherein a sixthmagnetic layer is located on a light entering side of the first magneticlayer and has a Curie temperature higher than the Curie temperature,T_(c1), of the first magnetic layer.
 17. The magneto-optical storagemedium as set forth in claim 16, wherein the sixth magnetic layer hassuch magnetic properties that the sixth magnetic layer is in an inplanemagnetization state in regions where the first magnetic layer has lostordered magnetization at temperatures that exceed T_(c1).
 18. Themagneto-optical storage medium as set forth in claim 2, wherein theCurie temperature, iT_(c3), of the third magnetic layer is higher thanthe Curie temperature, T_(c1), of the first magnetic layer.
 19. Themagneto-optical storage medium as set forth in claim 16, wherein thesub-lattice magnetization of rare-earth metal and transition metalscomposing the first magnetic layer and the sixth magnetic layer share acommon polarity.
 20. The magneto-optical storage medium as set forth inclaim 16, wherein a transparent dielectric layer is provided between thefirst magnetic layer and the sixth magnetic layer to block exchangecoupling and establish magnetostatic coupling between the first magneticlayer and the sixth magnetic layer.
 21. The magneto-optical storagemedium as set forth in claim 8, wherein an interference layer is locatedon the light entering side of the first magnetic layer, the interferencelayer being made of a material that, when used together with thetransparent dielectric layer, causes multi-interference of light. 22.The magneto-optical storage medium as set forth in claim 8, furthercomprising a reflective layer provided between the first magnetic layerand the transparent dielectric layer.
 23. An information reproducingmethod for reproducing information from a magneto-optical recordingmedium formed of a film and having a plurality of information tracks,each information track including in the following order; a firstmagnetic layer, a fourth magnetic layer, a third magnetic layer, afourth magnetic layer, a protection layer, wherein the first magneticlayer produces a smaller frictional force due to wall coercivity than dothe other magnetic layers, the second magnetic layer has a Curietemperature lower than do the other magnetic layers, the fourth magneticlayer is a perpendicular magnetization film producing a higher coerciveforce at room temperature than does the first magnetic layer, the thirdmagnetic layer is located between the second magnetic layer and thefourth magnetic layer, the third magnetic layer is in a perpendicularmagnetization state at least at temperatures that are in a proximity ofthe Curie temperature and changes into an in-plane magnetization stateat temperatures that are above the proximity of the Curie temperature,and the first, second, third, and fourth magnetic layers are mutuallycoupled through an exchange force, said method comprising the steps of:rotating the medium; projecting a light beam onto the rotating mediumfrom the side of the first magnetic layer; forming a temperaturedistribution having a gradient in the direction in which the light beammoves relative to the medium and having a region of temperatures higherthan the Curie temperature of the second magnetic layer; moving a domainwall in the first magnetic layer which comes into the region oftemperatures when the medium rotates toward a side having highertemperature in the temperature gradient in the first magnetic layer; andreproducing the information by detecting rays of the light beam.
 24. Aninformation reproducing apparatus for reproducing information from amagneto-optical recording medium formed of a film and having a pluralityof information tracks, each information track including in the followingorder, a magnetic domain expansion layer, a first intermediate layer, amagnetic masking layer, a recording layer, a protection layer, whereinthe recording layer has a plurality of storage magnetic domains, thefirst intermediate layer cuts off exchange coupling with the recordinglayer when temperature rises past a predetermined temperature, themagnetic domain expansion layer has magnetic walls which move toward ahigher temperature portion so as to form expanded magnetic domains whenthe first intermediate layer cuts off the exchange coupling with therecording layer, and the magnetic masking layer is located between therecording layer and the first intermediate layer, and is coupled withthe recording layer through an exchange force at temperatures that arenot higher than the predetermined temperature, and meanwhile cuts offmagnetostatic coupling of the magnetic domain expansion layer with therecording layer at temperatures that are higher than the predeterminedtemperature, said apparatus comprising: means for rotating the medium;means for projecting a light beam onto the rotating medium from the sideof the magnetic domain expansion layer; means for forming a temperaturedistribution having a gradient in the direction in which the light beammoves relative to the medium and having a region of temperatures higherthan the Curie temperature of the first intermediate layer; means formoving a domain wall in the magnetic domain expansion layer which comesinto the region of temperatures when the medium rotates toward a sidehaving higher temperatures in the temperature gradient in the magneticdomain expansion layer; and means for reproducing the information bydetecting rays of the light beam; and heating means for heating themedium and forming the temperature distribution, said heating meansbeing provided separately from the light beam projecting means.
 25. Aninformation reproducing apparatus for reproducing information from amagneto-optical recording medium formed of a film and having a pluralityof information tracks, each information track including in the followingorder; a first magnetic layer, a second magnetic layer, a third magneticlayer, a fourth magnetic layer, a protection layer, wherein the firstmagnetic layer produces a smaller frictional force due to wallcoercivity than do the other magnetic layers, the second magnetic layerhas a Curie temperature lower than do the other magnetic layers, thefourth magnetic layer is a perpendicular magnetization film producing ahigher coercive force at room temperature than does the first magneticlayer, the third magnetic layer is located between the second magneticlayer and the fourth magnetic layer, the third magnetic layer is in aperpendicular magnetization state at least at temperatures that are in aproximity of the Curie temperatures and changes into an in-planemagnetization state at temperatures that are above the proximity of theCurie temperature, and the first, second, third, and fourth magneticlayers are mutually coupled through an exchange force, said apparatuscomprising: means for rotating the medium; means for projecting a lightbeam onto the rotating medium from the side of the first magnetic layer;means for forming a temperature distribution having a gradient in thedirection in which the light beam moves relative to the medium andhaving a region of temperature higher than the Curie temperature of thesecond magnetic layer; means for moving a domain wall in the firstmagnetic layer which comes into the region of temperatures when themedium rotates toward a side having higher temperature in thetemperature gradient in the first magnetic layer; and means forreproducing the information by detecting rays of the light beam; andheating means for heating the medium and forming the temperaturedistribution, said heating means being provided separately from saidlight beam projecting means.
 26. The magneto-optical storage medium asset forth in claim 8, wherein the reproduction layer and the magneticdomain expansion layer are made of a rare-earth metal and transitionmetal respectively, and have the same polarity of sub-latticemagnetization of at least at temperatures ranging from room temperatureto a Curie temperature of the magnetic domain expansion layer.
 27. Themagneto-optical storage medium as set forth in claim 9, furthercomprising a reflective layer provided between the transparentdielectric layer and the magnetic domain expansion layer.